1. Field of the Invention
This invention concerns panes, especially panes covered with thin films produced by vacuum techniques, such as cathodic sputtering, and optical properties which vary as a function of the angle of incidence of the light.
2. Discussion of the Background
In many cases it would be desirable to have panes, the optical and energy properties of which vary as a function of the direction of illumination or observation. Conventionally, such an effect is achieved by shutters or Venetian blinds, but there are aesthetic or practical limits to these systems. In particular, they are not at all well adapted to automobiles.
However, it is particularly in modern automobiles that the need for such systems is increasingly becoming felt. The glazed parts are becoming ever larger in area and are more and more steeply inclined and therefore, offer very large areas for solar intake, especially in summer when the sun is high. This leads to overheating of the passenger space. The usual procedures for reducing the transmission, by increasing the reflection or the absorption of the panes unfortunately reach their limits very rapidly, because of the regulations which impose minimum values for the light transmission for the panes of an automobile, of whatever kind they may be. It therefore appears particularly advantageous to be able to have available a pane which assures the driver of an automobile, the required light transmission, but will allow the reduced transmission-especially the energy transmission--in other directions.
The need for a variable transmission pane is, however, not limited to automobiles, but in building construction as well. In residential buildings and offices, it is desired to provide protection against sunshine, but nevertheless to maintain the maximum contact with the outside world. Likewise, a need exists for functional panes which favor natural lighting, in particular in professional and business premises, especially in offices of the "landscaped" type, that is to say in office blocks of great depth. It is then desired to increase the illumination at the back of the rooms but to maintain a supportable level in the region of the windows. It is therefore a matter of combatting the natural decrease in illumination with distance from the glazed wall.
In the past, special case glasses with a prismatic structure have been developed for fulfilling this function, but apart from the fact that they interfere with a clear view of the external environment, they become dirty because their very pronounced relief surface traps dust. Moreover, the rolling techniques which enable them to be produced are difficult when it is desired to achieve a precise optical effect.
The invention therefore has the objective of providing a pane, the light and energy transmission of which varies as a function of the angle of incidence of the light. It is also necessary that such a pane can be produced industrially, that is to say particularly in dimensions compatible with use in applications such as automobile production or building with a satisfactory quality, that is in particular an identity of performance between different points of the pane.
A holographic technique has been proposed for supplying automobile panes which possess, for the driver, the required light transmission but which have a strong reflection for solar radiation when it meets the pane at a different angle of incidence. In particular, German Patent Applications DE 31,36946 and DE 38,22814 propose methods for the production of a holographic film to be integrated into an automobile pane. These techniques, though attractive from the theoretical standpoint and enabling the production of satisfactory prototypes, are not yet sufficiently developed to arrive at economical products which could become widespread in use.
In the laboratory, furthermore, means are known for creating transparent substrates, such as glass, with thin films deposited thereof having a "columnar" structure with a direction of the axis of the columns different from the perpendicular to the surface. These structures, are achieved over small areas most commonly of the order of one square centimeter, use vacuum deposition techniques.
Thus, in the article "Angular selective window coatings: theory and experiment" by MBISE, SMITH, NIKLASSON and GRANQVIST, SPIE vol. 1149, Optical materials technology for energy efficiency and solar energy conversion VIII (1989) pp 179-199, a method of thermal evaporation of silver and chromium is proposed, the evaporation source (the metal placed in a crucible) being punctiform and the different incidences all obtained on the same substrate situated 30 cm from the source. The specimens obtained possess, at the positions corresponding to a given oblique direction of deposition (for example 80.degree.), a light transmission that varies according to the direction of the incident light. Such a thermal evaporation technique, with one or more punctiform sources, cannot be used industrially for the production of columnar films, because with respect to each point of the specimen, there corresponds a different deposition direction and therefore not only a different perpendicular light transmission but also a different law of dependence upon the angle of incidence.
J. W. PATTEN in "The influences of surface topography and angle of adatom incidence on growth structure in sputtered chromium", Thin Solid Films, 63 (1979) pp 121-129, describes a cathodic sputtering technique in which a diaphragm (diameter 2.5 cm) is placed in front of the target. The substrate has facets orientated at six different angles from 0.degree. to 99.degree., which makes possible the obtaining of specimens of films having an area of at most 1 cm.sup.2. The measurements performed on the columnar films are not of an optical nature.
Thus the techniques of the prior art, if they enable the basic phenomena to be studied, did not provide any solution to the problem of the industrial production of columnar films.
Also known are means for stopping down the cathodic sputtering cathodes by placing, in front of the targets, masks of perforated plate, made either of expanded metal or of metal cloths, but their purpose is to obtain films of variable thicknesses, the variation of the ratio between the holes and the solid parts of the screens enabling the thickness of the films obtained in defined conditions to be varied.
Other techniques have been proposed for selecting, from among the particles emitted by the target with which the cathode of a sputtering apparatus is equipped, those that are emitted substantially perpendicularly to the surface of the target. Thus the Patent DD 285,897 proposes to place, before the target, a series of mutually parallel blades perpendicular to its surface (and to the axis of the cathode). The objective is positively to control the thickness of the deposits on objects of round shape.